The invention concerns devices for controlling the pitch of rotor aircraft rotor blades, particularly a helicopter main rotor, the rotor being of the type for which each blade is, on the one hand, rotated around an axis of rotation of a rotor shaft, or rotor axis by means of a hub rotating integrally with the shaft, and, on the other hand, rotating integrally, around a longitudinal blade pitch change axis, with at least one pitch lever controlled by a corresponding pitch rod.
The invention relates more exactly to a pitch control device of the type including a swash-plate assembly, and such that each pitch rod is connected to a plate rotating with the rotor and belonging to the swash-plate assembly, in which the rotating plate is mounted to rotate on a non-rotating plate, restrained against any rotation around the rotor axis, the two plates being annular, centered on the rotor axis and surrounding this axis and being able to be translated axially, i.e. parallel to the rotor axis, and tilted in any direction around the rotor axis, being activated by control actuators connecting the non-rotating plate to the rotor aircraft structure for the control of the collective pitch and the cyclic pitch respectively of the blades.
The control device of the invention is in particular of the type in which, moreover, the non-rotating plate is restrained against rotation by means of two non-rotating pillars around the rotor axis, which are parallel to said rotor axis, diametrically opposite in relation to this axis, and connected to the structure, and which guide the plates in translation along the rotor axis by means of two movable slides each along a respective pillar and belonging to an articulated connection which connects the non-rotating plate to the pillars and allows the plates to be tilted in any direction around the rotor axis by pivoting of the non-rotating plate around two diametrical axes of the latter, which are perpendicular to each other and one of which remains perpendicular to the rotor axis, each of the two slides cooperating with one of two coaxial trunnions respectively around a first of the two pivoting diametrical axes.
Such a pitch control device, known generally via DE-A-36 20 794, enables the height of a rotor shaft-hub assembly and swash-plate device to be reduced in comparison with conventional embodiments, in which the swash-plates are able to be translated axially and to be tilted by an axial translation and tilting guidance mechanism which includes a central ball joint, centered on the rotor axis, and on which the non-rotating plate, and therefore the swash-plates, are oscillating mounted, the ball joint being itself mounted sliding axially (parallel to the rotor axis) around a cylindrical guide coaxial to the rotor axis and not rotating around this rotor axis, and generally fixed in relation to the rotor aircraft structure. When the swash-plates surround the rotor shaft, which is most often the case, the cylindrical guide is tubular, surrounds the rotor shaft, and is fixed to a housing integral with the structure of the rotor aircraft and surrounding the connection between the rotor shaft base and the main transmission box.
In these conventional embodiments, one or more rotating and upper or non-rotating and lower connection unit or units, which respectively connect the rotor to the rotating plate to rotate the latter, and the structure to the non-rotating plate, to restrain the latter from any rotation around the rotor axis, is or are one or more scissor-articulated torque links with two arms connected to each other by a pivot or a hinge, which enables the two arms of each torque link to move apart or draw together in order to enable movements in the direction of the rotor axis, since the upper and lower arms of a rotating torque link are moreover articulated on the shaft or the hub of the rotor and on the rotating plate respectively, and since the lower and upper arms of a non-rotating torque link are moreover articulated on the structure of the rotor aircraft and on the non-rotating plate.
The drawbacks of these rotating and non-rotating torque links are that their articulations comprise traditional, unreliable, bearings, or, on more recent helicopters, ball joints and self-lubricating bearings including numerous carbide pins and rings, which are heavy and expensive.
Furthermore, a certain overall height or axial requirement (parallel to the rotor axis) and requirement in width is necessary for the torque links to operate correctly. So, to facilitate their loading for example in the hold of a transport aircraft and to improve their capacity to be accommodated in a hangar, for example of a ship, modern helicopters must offer the best possible vertical or axial compactness of the shaft and the main rotor assembly and of the swash-plate assembly which surrounds it. But reduction in the height or axial size of such an assembly is limited by the interference between the two arms of a torque link during folding of the latter and by the maximum swiveling angles permitted by the construction of the articulation joints of the torque links.
The axial space requirement of a swash-plate control device with torque links being mainly determined by the clearance of the torque links, on account of the folding and swiveling angles necessary for their operation, a proposal has already been made, in order to reduce the height of such an assembly, to eliminate either the rotating torque link or links, or the non-rotating torque link or links, and to use two guide pillars, parallel to each other and to the rotor axis and on either side of this axis in the same radial plane passing through the rotor axis, of a guidance mechanism for the axial translation and tilting of the swash-plates which also includes a cardan joint connection, with an intermediate ring mounted sliding axially by means of the pillars, and articulated pivoting on these pillars around a diametrical axis of the ring, which is an axis remaining perpendicular to the rotor axis, whereas the ring is articulated pivoting on one of the swash-plates around another diametrical axis of the ring, which is perpendicular to the first diametrical axis, as proposed in DE-A-36 03 400 and DE-A-36 20 794. Thus, axial movements of the swash-plates and of the ring are guided by the two pillars, whereas tilting movements of the swash-plates in any direction around the rotor axis are obtained by pivoting the swash-plates in relation to the ring around one of two diametrical and perpendicular axes of the ring, and by pivoting the ring on the pillars around the other of these two perpendicular diametrical axes.
The guide pillars can be rotating (rotating integrally with the rotor), and so too can the ring mounted in this case between the rotating pillars, on the one hand, and, on the other hand, the rotating plate thus rotated by the pillars and the ring which replace the rotating torque link or links. But the guide pillars can also be non-rotating, i.e. restrained against any rotation around the rotor axis by a support fixed to the structure of the rotor aircraft, in which case the ring is also non-rotating and mounted between, on the one hand, the non-rotating pillars, and, on the other hand, the non-rotating plate, thus restrained from rotating by the non-rotating pillars and the non-rotating ring, which replace the non-rotating torque link or links.
If the pillar and cardan ring devices have the advantage of providing a saving in overall height (axial) requirement compared with torque link devices, pillar devices do nonetheless have the drawbacks of a greater transverse space requirement, due to the greater diameter of the swash-plate assembly in order to house the intermediate ring between the swash-plates and the pillars, and low resistance and poor control of vibration, which leads to the use of rotating pillars being avoided. Lastly, the use of rotating or non-rotating guide pillars does not prevent the simultaneous use of one or more respectively non-rotating or rotating torque links, since a device comprising both non-rotating pillars and rotating pillars must be ruled out, in so far as it does not allow all the tilting movements required by the cyclic pitch controls and provided by the cardan ring.
Furthermore, in the device with non-rotating pillars of DE-A-36 20 794, each of the two slides is arranged as a crosspiece, one branch of which slides axially without notable play and the other branch of which constitutes the trunnion allowing the pivoting, by one of two diametrically opposite bearings, of the cardan ring on the pillars around a diametrical axis of the non-rotating plate, the ring being itself pivoting on the non-rotating plate, around the other diametrical and perpendicular axis, by two ball joints in two other diametrically opposite bearings.
The problem at the basis of the invention is to improve a swash-plate device for controlling pitch which comprises non-rotating pillars for restraining rotation of the non-rotating plate, so that it does not comprise two pairs of articulations with bearings diametrically opposite two by two, nor crosspieces, which are fragile components, expensive and difficult to manufacture, assemble and maintain, to allow translation and tilting movements of the rotating plates.